1. Field of the invention
The present invention relates to a method of forming charge-transfer thin films, more particularly relates to a method of forming organic superconducting or conducting thin films.
2. Description of the Prior Art
Since 1980 in which (TMTSF).sub.2 PF.sub.6 was reported for the first time as a superconducting material made from an organic substance having a critical temperature of 0.9K, several charge-transfer complexes have been confirmed to be superconducting, e.g. those comprising donor molecules such as TMTSF (tetramethyl-tetraselenefulvalene), TMTTF (tetramethyl-tetrathiafulvalene), BEDT-TTF (BEDT-TTF: Bis-(ethylenedithio)-tetrathiafulvalene) and those comprising acceptor molecules such as Ni(dmit). The critical temperature of (BEDT-TTF).sub.2 Cu(NCS).sub.2 reaches as high as 11.0K for example. The organic superconducting materials of this kind are sorted out in first and second types. Superconducting materials of the first type are such that orbitals of p.pi. electrons are superimposed between molecules to form a column structure in a quasi-one dimensional structure. Superconducting materials of the second type are such that the anisotropy in molecular arrangement and the anisotropy in molecular orbital are readily balanced in order to form a quasi-two dimensional structure. Both types are characterized by anisotropy in a reduced dimensional structure. The critical temperatures of superconducting materials of the first type (one-dimensional) vary by application of pressure. For example, while Tc remains between 1.0K and 1.5K in the case of .beta.-(BEDT-TTF).sub.2 I.sub.3 just after prepared, it has been reported that, when a pressure was applied to and then removed from the complex, the Tc was elevated by 7K.
These charge-transfer organic complexes exhibiting superconductivity are synthesized by methods utilizing solvents represented by diffusion or electrolysis methods. The sizes of crystals obtained by these methods, however, are very small and therefore it is difficult to use them for electronic appliances. Furthermore, at least one axes of respective crystals have to be aligned in a common direction in order to realize desired superconductivity as required for superconducting devices. Hitherto, several attempts such as utilizing evaporation, have been made to form organic superconducting thin films. The determination of deposition conditions for thin film formation of charge-transfer complexes become very difficult when the orientation of molecules has to be taken into consideration. In fact, the epitaxial growth of an organic charge-transfer complex can not easily take place on a substrate by procedures employed in conventional processes, unlike the conventional deposition of an inorganic film on an inorganic substrate, so that many deposition conditions such as the distance between the substrate to be coated and the target have to be simultaneously satisfied in order to achieve desired alignment of molecules of the charge-transfer complex so that reproductivity can hardly be obtained.